Well, Photoecosse gets the prize. Just to see what it did this time, this morning I launched Lr5 and tried to open an image into Silver Efex. This time, Silver Efex opened without having to go through CC, but with the blank image, as before. I went to the settings to see if there was anything that might be useful to adjust and looked at the GPU setting again. This time the I was able to deselect 'enable GPU' and did not get an error message saying that GPU processing was unavailable. I closed the box, closed Silver Efex and re-opened it from Lr. Worked perfectly! I checked the other Nik applications and they are now working as well.
This makes no sense but it's fixed. I'll settle for that! Thanks all for your help.